The present invention relates generally to magnetic resonance imaging (“MRI”) systems and methods and, more particularly, the invention relates to systems and methods for producing images using parallel imaging using specialized masking techniques tailored to regions of interest within the subject of the parallel imaging study.
Magnetic resonance imaging (“MRI”) uses the nuclear magnetic resonance (“NMR”) phenomenon to produce images. When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the nuclei in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) that is in the x-y plane and that is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped,” into the x-y plane to produce a net transverse magnetic moment Mxy. A signal is emitted by the excited nuclei or “spins,” after the excitation signal B1 is terminated, and this signal may be received and processed to form an image.
When utilizing these “MR” signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received MR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
The measurement cycle used to acquire each MR signal is performed under the direction of a pulse sequence produced by a pulse sequencer. Clinically available MRI systems store a library of such pulse sequences that can be prescribed to meet the needs of many different clinical applications. Research MRI systems include a library of clinically-proven pulse sequences and they also enable the development of new pulse sequences.
The MR signals acquired with an MRI system are signal samples of the subject of the examination in Fourier space, or what is often referred to in the art as “k-space.” Each MR measurement cycle, or pulse sequence, typically samples a portion of k-space along a sampling trajectory characteristic of that pulse sequence. Most pulse sequences sample k-space in a raster scan-like pattern sometimes referred to as a “spin-warp,” a “Fourier,” a “rectilinear,” or a “Cartesian” scan. The spin-warp scan technique employs a variable amplitude phase encoding magnetic field gradient pulse prior to the acquisition of MR spin-echo signals to phase encode spatial information in the direction of this gradient. In a two-dimensional implementation (“2DFT”), for example, spatial information is encoded in one direction by applying a phase encoding gradient, Gy, along that direction, and then a spin-echo signal is acquired in the presence of a readout magnetic field gradient, Gx, in a direction orthogonal to the phase encoding direction. The readout gradient present during the spin-echo acquisition encodes spatial information in the orthogonal direction. In a typical 2DFT pulse sequence, the magnitude of the phase encoding gradient pulse, Gy, is incremented, ΔGy, in the sequence of measurement cycles, or “views” that are acquired during the scan to produce a set of k-space MR data from which an entire image can be reconstructed.
Magnetic resonance angiography (MRA) uses the magnetic resonance phenomenon to produce images of the human vasculature. To enhance the diagnostic capability of MRA a contrast agent such as gadolinium can be injected into the patient prior to the MRA scan. When using a contrast agent, such vascular imaging is typically referred to a contrast enhanced MRA (CE-MRA or CEMRA). In practice, a moderate amount (10-30 ml) of a gadolinium-based contrast agent is typically injected into an arm vein. The contrast material then mixes with the systemic blood in the heart and pulmonary vasculature and passes from the left heart into the arterial circulation. The presence of contrast material in the blood causes the net T1 relaxation time to be altered from its unenhanced value, for example, of about 1000 msec to values in the range, for example, of 50 to 100 msec. MR acquisition methods can exploit this change in T1, causing the enhanced blood within the vasculature to be significantly brighter compared to other structures within the imaging FOV.
There are a wide variety of technical challenges to performing CE-MRA to yield the desired information for a particular setting. As described in U.S. Pat. No. 5,417,213 the trick with this CE-MRA method is to acquire the central k-space views at the moment the bolus of contrast agent is flowing through the vasculature of interest. Collection of the central lines of k-space during peak arterial enhancement is key to the success of a CE-MRA exam. If the central lines of k-space are acquired prior to the arrival of contrast, severe image artifacts can limit the diagnostic information in the image. Alternatively, arterial images acquired after the passage of the peak arterial contrast are sometimes obscured by the enhancement of veins. In many anatomic regions, such as the carotid or renal arteries, the separation between arterial and venous enhancement can be as short as 6 seconds. However, this timing constraint is in opposition with the need to obtain a high spatial resolution image, for example, a three-dimensional (3D) image with adequate spatial resolution. To do so, it is necessary to have a sufficiently long acquisition time, generally in the range of ten seconds or longer, in order to collect enough information to yield the desired spatial resolution
The short separation time between arterial and venous enhancement dictates the use of acquisition sequences of either low spatial resolution or very short repetition times (TR). Short TR acquisition sequences severely limit the signal-to-noise ratio (SNR) of the acquired images relative to those exams in which longer TRs are possible. The rapid acquisitions required by first-pass CE-MRA methods thus impose an upper limit on either spatial or temporal resolution.
As a result, depending on the technique used and the trade-offs that may be tolerated in a given clinical setting, it may be possible to utilize one of a variety of different strategies that have been developed to shorten the scan time. For example, one such strategy is referred to generally as “parallel MRI” (“pMRI”). Parallel MRI techniques use spatial information from arrays of radio frequency (“RF”) receiver coils to substitute for the spatial encoding that would otherwise have to be obtained in a sequential fashion using RF pulses and magnetic field gradients, such as phase and frequency encoding gradients. Each of the spatially independent receiver coils of the array carries certain spatial information and has a different spatial sensitivity profile. This information is utilized in order to achieve a complete spatial encoding of the received MR signals, for example, by combining the simultaneously acquired data received from each of the separate coils.
Parallel MRI techniques allow an undersampling of k-space by reducing the number of acquired phase-encoded k-space sampling lines, while keeping the maximal extent covered in k-space fixed. The combination of the separate MR signals produced by the separate receiver coils enables a reduction of the acquisition time required for an image, in comparison to a conventional k-space data acquisition, by a factor generally bounded by the number of the receiver coils. Thus, the use of multiple receiver coils acts to multiply imaging speed, without increasing gradient switching rates or RF power.
Two categories of such parallel imaging techniques that have been developed and applied to in vivo imaging are so-called “image space methods” and “k-space methods.” An exemplary image space method is known in the art as sensitivity encoding (“SENSE”), while an exemplary k-space method is known in the art as simultaneous acquisition of spatial harmonics (“SMASH”). With SENSE, the undersampled k-space data is first Fourier transformed to produce an aliased image from each coil, and then the aliased image signals are unfolded by a linear transformation of the superimposed pixel values. With SMASH, the omitted k-space lines are synthesized or reconstructed prior to Fourier transformation, by constructing a weighted combination of neighboring k-space lines acquired by the different receiver coils. SMASH requires that the spatial sensitivity of the coils be determined, and one way to do so is by “autocalibration” that entails the use of variable density k-space sampling. A more recent advance to SMASH techniques using autocalibration is a technique known as generalized autocalibrating partially parallel acquisitions (“GRAPPA”), as described, for example, in U.S. Pat. No. 6,841,998. With GRAPPA, k-space lines near the center of k-space are sampled at the Nyquist frequency, in comparison to the undersampling employed in the peripheral regions of k-space. These center k-space lines are referred to as the so-called autocalibration signal (“ACS”) lines, which are used to determine the weighting factors that are utilized to synthesize, or reconstruct, the missing k-space lines.
When applied to CE-MRA acquisitions, short repetition time (TR) gradient echo sequences allow rapid collection of MRI data, and this can be accelerated with undersampling techniques such as SENSE and GRAPPA. Synchronizing the acquisition to the contrast arrival can be done using a test bolus or fluoroscopic triggering, such as described by Wilman A H, Riederer S J, King B F, Debbins J P, Rossman P J, Ehman R L. Fluoroscopically-triggered contrast-enhanced three-dimensional MR angiography with elliptical centric view order: application to the renal arteries. Radiology 1997; 205:137-146. An extension of the acquisition duration well into the venous phase, but with negligible venous signal, can be done using various centric phase encoding view orders, such as described by Wilman A H, Riederer S J. Performance of an elliptical centric view order for signal enhancement and motion artifact suppression in breathhold three dimensional gradient echo imaging. Magn Reson Med 1997; 38:793-802.
CE-MRA has been used successfully to image many vascular regions of the body, such as the intracranial, carotid, and renal arteries and the arteries of the peripheral vasculature. In particular, SENSE acceleration with typical acceleration applied along the two phase encoding directions has been useful in driving down the acquisition times in the brain, periphery, and abdomen, with 2D acceleration factors (R) as high as R=8. However, at this level of acceleration, the degree of loss of signal-to-noise ratio (SNR) due to the extensive level of undersampling, and the accompanying high level of aliasing, can become problematic.
It is well known that knowledge regarding an object that lies within the field of view (FOV) and what regions within the FOV might not contain any object can be used with traditional SENSE acquisitions to provide improved performance and temper the SNR loss. Such processes typically employ or are referred to “masking” techniques. Though particular masking techniques may vary slightly, masking techniques typically involve an attempt to determine the edges or borders of the object being imaged. Using these determined edges or borders, voxels falling outside of the borders are assumed to have no magnetization, and thus no MRI signal. Specifically, during the SENSE inversion process, these voxels are forced to have zero signal, which causes the algebraic inversion of the SENSE equations involving those zeroed voxels to be simplified, eliminates the spurious dispersion of signal to those voxels, and more properly assigns the measured signal to voxels actually located within the object.
Of course, the clinical suitability of such masking techniques is directly correlated to the accuracy of the determined edges or borders. However, the accuracy of the determined edges or borders is typically dependent upon the amount of time and information available to the clinician in making the determination. For example, highly accurate edges or borders could be determined by a clinician by acquiring a high-accurate anatomical image of the subject as a precursor to the SENSE-based acquisition. However, such an additional imaging acquisition would typically substantially extend the duration of the overall imaging process and, in some cases, defeat the purpose of attempting to accelerate the imaging process using SENSE.
It would therefore be desirable to provide a system and method for performing contrast enhanced MRA imaging studied employing parallel imaging without suffering clinically-unacceptable loss of SNR or a high level of aliasing, but without requiring clinicians to perform extensive pre-planning or additional imaging acquisitions that undermine the improvements in throughput that parallel imaging is typically selected to provide.